Natural selection is the a long-term process where species  with traits that help their species to survive and reproduce in their environment that they were living. Organism in that species that have the same specific  trait  will have a higher chance to survive and  more likely to pass those traits on to the next generation. After a long period of time, helpful traits become more common in a population. However, organism without the same trait will die quickly since they don't have the same trait that help them to survive in the same environment.  For example, if long necks giraffes can eat the leaves from the higher trees but shorter necks giraffes can't. Over time, all the giraffes will have long necks, but all the shorter ones will die because they couldn't find any food.